To: |
"'STEVEN & NANCY FRAASCH'" <sjfraasch@embarqmail.com>,"'Robert Bonner'" <rbonner@qro.com>, "'Tom W8JI'" <w8ji@w8ji.com> |
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Subject: |
Re: [Amps] Grid Dipping the Pi network in a new amp? |

From: |
"Chuck Curran" <ccurran@wi.rr.com> |

Date: |
Tue, 17 Jul 2007 21:22:56 -0500 |

List-post: |
<mailto:amps@contesting.com> |

To All: I have received quite a bit of welcomed advice, again, thanks to all who have taken the time to make suggestions and recommendations. A little more information was requested on this amp design, so I had better get to it. Just to repeat for any new readers, I tried grid dipping the tank circuit and it just won't dip. The amps is based on a pair of 3-400Z's, which I have owned since new in 1969 -- they ran in a 10 meter linear for one DX CW contest back in 1970, or about 48 hours. Then back in the box. All of the questions, I hope to answer with this description. I have a brand new National R-175A, as the plate choke, feeding into a pair of 500 pf doorknob caps. A 300 pf vacuum variable tune cap into the B&W 852 tank coil, then a 1300 pf vacuum variable with a padding cap to get up to 1800 pf on 80 meters. I have checked the tank coil using two impedance bridges, getting 6.9 and 7.1 uH against a B&W spec of 7.0 uH for the 80 meter inductance. All was checked for shorts to ground. Each cap was also checked, and they can be varied from minimum to maximum capacitance without any shorts or other negative items being detected. I will have 3,150 VDC no load, coming from a 5 Kw distribution transformer that was unwound down to 2150 VAC -- lots of fun. (That's another story) I am figuring about 2900 VDC under load, at 800 ma. So the resulting plate impedance, using the formula Rp = Ep/2*Ip gives around 1800 ohms. The specific B&W 852 tank coil was selected, since it was designed to work with a low voltage high current system, which I believe I have. Do the calculations yourself and you will find that this results in a design Q of 12, the good ol' magic number. I fully agree with the observations made by one of the poster, this tank coil would be bad news with a 4500 VDC plate voltage and low current. However, I feel it has a high probability of working , within it's intended work envelope. We will see soon, I hope! I have little time, the amp has been on the bench for more than 10 months, but this Saturday I intend to pull out the new MFJ-259B antenna analyzer and see what I can determine. First, I will make darn sure that all of the brass strap connecting the vacuum variables is making a proper connection, without any shorts etc. I make mistakes too, like 12 a day, just ask my wife! Again, Thanks for the generous thoughts and ideas, I will no doubt try them all this weekend! Chuck WA9POU -----Original Message----- From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On Behalf Of STEVEN & NANCY FRAASCH Sent: Tuesday, July 17, 2007 3:03 PM To: amps@contesting.com Subject: Re: [Amps] Grid Dipping the Pi network in a new amp? Bob: The antenna analyzer SHOULD have a source impedance of ~50 ohms. If it didn't it would cause a reflection (or possibly reduce or cancel a reflection) if it presents an odd conjugate condition other than 50 ohms real. Maybe I am not clear on what to do: You're attaching a load impedance to the input of the tank that emulates the desired tube operating load impedance. Chuck said that this is ~1800 ohms. Given that a resistor of that value is placed from that node (plate cap) to RF ground (along with the shunt capacitance of the tube's output, Cp-g for grounded grid), and the tank is properly adjusted, you should measure 50 ohms or a 1:1 SWR at the output of the amplifier at desired frequency. There is no other termination at the output, only the source impedance of the antenna analyzer, 50 ohms. You could go the other way, measure from the tank input and hang a 50 ohm termination at the amp output, but most impedance bridges are too inaccurate or won't resolve > 1K ohms. This whole thing reminds me a bit of WG0M in Minneapolis a few years ago who wanted me to design a matching network for his 160m "L" antenna. I used NEC2 to ballpark it. He hooked it up and was mad because he still had 3:1 SWR. I went over to help him, and it was immediately obvious what the problem was: The vertical portion of the "L" ran parallel and 3" away from his 60' tower. I never knew (nor did he tell me) he had a tower. A little fiddling and we had a match. So if there's anything you're forgetting to tell me Chuck ... 73, Steve, K0SF ----- Original Message ----- From: "Bob Sullivan" <robert@isquare.com> To: "STEVEN & NANCY FRAASCH" <sjfraasch@embarqmail.com> Sent: Tuesday, July 17, 2007 2:09:52 PM (GMT-0600) America/Chicago Subject: Re: [Amps] Grid Dipping the Pi network in a new amp? Steve, Been reading your very informative comments - thanks for taking the time to post. The idea of using an antenna analyzer is a great one but I thought about what you said below and was wondering if I should place a 52-ohm termination at the output of the tank before utilizing the analyzer (MFJ)? Thanks! 73, Bob WØYVA Personal site: http://www.isquare.com/personal_pages/ras-hardware.htm At 08:22 AM 7/17/2007, you wrote: > |

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